1. Field of the Invention
The present invention relates to an electroconductive, particle to form various types of electrodes such as sensor electrodes, heater electrodes and lead wire electrodes, and further to a metal paste for forming an electrode using the electroconductive particle.
2. Description of the Related Art
In manufacture of sensor electrodes of various types of gas sensors such as oxygen sensors, NOx sensors and exhaust gas temperature sensors, heater electrodes and the like, the manufacture is carried out generally by applying a metal paste containing an electroconductive metal powder on a substrate by one of various types of methods such as screen printing and firing the applied metal paste. A form of a metal paste is often used because the metal paste form is preferable from the viewpoint that the form can cope with a complicated pattern of an electrode, and besides from the viewpoint of the manufacturing efficiency in which a substrate and an electrode can simultaneously be manufactured by application and firing of a metal paste on a green sheet for forming a ceramic substrate.
As a metal paste for forming an electrode, a metal paste is conventionally used in which an electroconductive particle such as a precious metal and a ceramic powder such as Al2O3 or ZrO2 are mixed in a solvent. Mixing a ceramic powder in a metal paste, when a substrate and an electrode are manufactured by application and firing of a metal paste on a green sheet as described above, is for improving the adherence of the electrode by correcting a difference in contraction rate between the metal paste and the green sheet and eliminating the problem of warpage and deformation of the substrate due to the contraction rate difference. Further, mixing a ceramic powder in a metal paste also exhibits an advantage of being capable of preventing oversintering of the electroconductive particle in firing.
However, a ceramic powder, while securing formability of an electrode film as described above, raises the resistance value of the manufactured electrode film, and is likely to make the resistance value considerably higher than in an electrode of a bulk metal. Therefore, the use of a ceramic powder is not preferable from the viewpoint of the property as a precursor material of an electrode, however if no ceramic or a too small amount thereof is mixed, the formation itself of an electrode becomes impossible, and thereby there has been no choice but to mix a ceramic as an actual situation.
The significance of mixing a ceramic powder in a metal paste for forming an electrode has also an aspect of securing the durability of an electrode film. The durability of an electrode is a property required, for example, in an electrode film exposed to a high temperature of a heater electrode or the like, and an electrode film having poor durability has a risk of causing wire breakage in a relatively short time. Then, although the durability of an electrode film can be improved by an increase in the amount of a ceramic powder mixed, there is a demand of reducing the amount of the ceramic powder mixed from the viewpoint of the reduction of the resistance value, as described above.